Passive detection of pulse

ABSTRACT

A body-mountable light sensing device includes a photodiode configured to receive light from a portion of subsurface vasculature and electronics configured to operate the photodiode to measure the received light. The electronics include a photodiode voltage source configured to reverse bias the photodiode, a current mirror, and a sigma-delta modulator configured to generate a digital output related to the received light and having a high resolution while using low power. The digital output could be used to determine a pulse rate or other properties of blood in the portion of subsurface vasculature by detecting absorption of ambient light by blood in the portion of subsurface vasculature. Components of the body-mountable device could be embedded in a polymeric material configured for mounting to a surface of an eye. The digital output and/or related information could be wirelessly communicated by the body-mountable device.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Optical properties of blood (e.g., absorptivity at a specifiedwavelength(s)) can differ from optical properties of surrounding tissue(e.g., skin, the walls of blood vessels); further, optical properties ofblood can be related to other properties of the blood (e.g., the oxygenbinding state of hemoglobin in the blood). Thus, the presence and/oramount of blood in a tissue, the oxygen saturation of the blood, orother properties of blood and/or tissue can be detected by illuminatingthe blood and/or tissue and detecting a property of light reflected,refracted, transmitted, scattered, or otherwise emitted by the bloodand/or tissue in response to the illumination. A measurement of thevolume of blood in a tissue over time (e.g., by measuring the degree towhich the tissue and/or blood absorb an illuminating light over time)can be used to determine a pulse rate, a flow profile, a pressureprofile, or some other information about perfusion and/or flow of bloodin the tissue. A measurement of the amount of illumination that isabsorbed by blood at two or more specified wavelengths can allow thedetermination of an oxygen saturation of the blood.

SUMMARY

Some embodiments of the present disclosure provide a body-mountabledevice including: (i) a photodiode, wherein the photodiode is configuredto detect light received from a portion of subsurface vasculature,wherein the light received from the portion of subsurface vasculature isrelated to blood in the portion of subsurface vasculature; (ii) aphotodiode voltage source, wherein the photodiode voltage source isconfigured to apply a voltage to the photodiode such that the photodiodeis reverse biased, wherein a current through the photodiode is relatedto the light received from the portion of subsurface vasculature; (iii)a sigma-delta modulator, wherein the sigma-delta modulator is configuredto receive an input and to provide a digital output related to theinput; and (iv) a current mirror, wherein the current mirror isconfigured to provide an output current that is related to the currentthrough the photodiode, wherein the input received by the sigma-deltamodulator is the output current of the current mirror.

Some embodiments of the present disclosure provide a method including:(i) mounting a body-mountable device to an external body surfaceproximate to a portion of subsurface vasculature, wherein thebody-mountable device includes (A) a photodiode, wherein the photodiodeis configured to detect light received from the portion of subsurfacevasculature, wherein the light received from the portion of subsurfacevasculature is related to blood in the portion of subsurfacevasculature, (B) a photodiode voltage source, wherein the photodiodevoltage source is configured to apply a voltage to the photodiode suchthat the photodiode is reverse biased and such that a current throughthe photodiode is related to the light received from the portion ofsubsurface vasculature, (C) a sigma-delta modulator, wherein thesigma-delta modulator is configured to receive an input and to provide adigital output related to the input, and (D) a current mirror, whereinthe current mirror is configured to provide an output current that isrelated to the current through the photodiode, wherein the inputreceived by the sigma-delta modulator is the output current of thecurrent mirror; (ii) operating the body-mountable device to generate thedigital output; and (iii) determining a property of blood in the portionof subsurface vasculature based on the generated digital output.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system that includes aneye-mountable device in wireless communication with an external reader.

FIG. 2A is a bottom view of an example eye-mountable device.

FIG. 2B is an aspect view of the example eye-mountable device shown inFIG. 2A.

FIG. 2C is a side cross-section view of the example eye-mountable deviceshown in FIGS. 2A and 2B while mounted to a corneal surface of an eye.

FIG. 2D is a side cross-section view enhanced to show the tear filmlayers surrounding the surfaces of the example eye-mountable device whenmounted as shown in FIG. 2C.

FIG. 2E is a front view of the example eye-mountable device shown inFIGS. 2C and 2D while mounted to a corneal surface of an eye.

FIG. 2F is a front view of the example eye-mountable device shown inFIGS. 2C and 2D while mounted to a corneal surface of an eye.

FIG. 3 is a functional block diagram of an example system for detectinglight from a portion of subsurface vasculature.

FIG. 4 illustrates an example circuit for generating a digital outputrelated to light received from a portion of subsurface vasculature.

FIG. 5 is a flowchart of an example process for operating a light sensorin a body-mountable device to detect light received from a portion ofsubsurface vasculature.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying figures, which form a part hereof. In the figures, similarsymbols typically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, figures, and claims are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the scope of the subject matter presented herein. It willbe readily understood that the aspects of the present disclosure, asgenerally described herein, and illustrated in the figures, can bearranged, substituted, combined, separated, and designed in a widevariety of different configurations, all of which are explicitlycontemplated herein.

I. Overview

Embodiments provided herein include a body-mountable device configuredto optically detect a property of blood in a portion of subsurfacevasculature (e.g., a volume of blood, a content and/or saturation of oneor more gasses in the blood) by detecting light (e.g., visible light,infrared light, ultraviolet light) received from the portion ofsubsurface vasculature (e.g., detecting an intensity, a polarization, acolor, a spectral profile of the received light) in response toillumination of the portion of subsurface vasculature by ambient light(e.g., direct or indirect sunlight, light from a lamp or lightingfixture, light from a display, light from some other ambient light inthe environment of the portion of subsurface vasculature) and/or bylight emitted by a light source in the body-mountable device. Opticaldetection of the light received from the portion of subsurfacevasculature could include reverse-biasing a photodiode configured toreceive the light from the portion of subsurface vasculature and using acurrent-mode sigma-delta modulator to generate a digital output relatedto a current through the reverse-biased photodiode that is related to aproperty of the light received by the photodiode. Further, a currentmirror could be used to present a current to the sigma-delta modulatorthat is related to the current through the photodiode.

In some embodiments, the body-mountable device is configured to detect avolume of blood in the portion of subsurface vasculature (e.g., to allowphotoplethysomographic application(s)). For example, the body-mountabledevice could be configured and/or operated to detect light received fromthe portion of subsurface vasculature that is related to the absorptionof ambient light by blood in the portion of subsurface vasculature thatis, in turn, related to the volume of blood in the portion of subsurfacevasculature. Such volume or other measurements could be performed usingthe body-mountable device a plurality of times, and a plurality ofrespectively detected blood volumes or other properties could be used todetermine a flow rate, a pulse rate, a flow profile, or some otherinformation about the blood in the portion of subsurface vasculatureand/or about a health state of a human to which the body-mountabledevice is mounted.

In some embodiments, the body-mountable device could include a furtherphotodetector (e.g., another photodiode and sigma-delta modulator,and/or a second photodiode connected to a first sigma-delta modulatorthat is also connected to the first photodiode) configured to detectanother property of light received from the portion of subsurfacevasculature and/or from another portion of subsurface vasculature. Sucha second photodetector could be used to determine some property of ahuman to which the body-mountable device is mounted. For example, thephotodiode could be configured to detect the intensity of received lightwithin a first range of wavelengths, a further photodetector of thebody-mountable device could be configured to detect the intensity ofreceived light within a second range of wavelengths, and an oxygensaturation or other property of a gas content of blood from which thelight was received could be determined based on the detectedintensities. Additional properties of blood in the portion of subsurfacevasculature and/or other applications of a body-mountable device asdescribed herein are anticipated.

Variations in or other properties of light received from a portion ofsubsurface vasculature in response to illumination of the portion ofsubsurface vasculature by ambient light could have very small values,and a related current and/or change in current through the photodiodecould have a very small amplitude. Thus, the sigma-delta modulator couldbe configured to have a very high current resolution. For example, thesigma-delta modulator could be configured and/or operated to have acapacitor sink current, a comparator clock frequency, or some otherproperty or combination of properties such that the digital output ofthe sigma-delta modulator contains information related to the currentthrough the photodiode that has a resolution that is less thanapproximately 100 picoamps. Further, the photodiode, sigma-deltamodulator, and related components (e.g., a feedback amplifier configuredto apply a specified voltage to the photodiode, a current mirrorconfigured to apply a current to the sigma-delta modulator that issubstantially equal to the current through the photodiode) could beconfigured to operate using very little power (e.g., less thanapproximately 500 nanoamps) according to the constraints of anapplication (e.g., a wearable or otherwise battery-powered application,an application powered by wireless energy, an application powered byambient energy harvested form the environment of the body-mountabledevice).

Body-mountable devices as described herein could take a variety of formsand be configured in a variety of ways to be mounted on body locationsaccording to an application. Such devices could include mounts (e.g.,straps, adhesives) or other features (e.g., a geometry configured toconform to and/or around an element of human anatomy) configured toposition the body-mountable device relative to a portion of subsurfacevasculature such that the photodiode can receive light from the portionof subsurface vasculature. The photodiode could receive the receivedlight directly, or through one or more filters, mirrors, lenses,diffraction gratings, and/or other optical elements. Body-mountabledevices could be powered by a battery, a tether, or some other method.In some examples, a body-mountable device could be powered by radiatedenergy received at the body-mountable device. Such received radiatedenergy could be rectified and/or regulated in real time to power thebody-mountable device. For example, power can be generated from incidentradio frequency radiation inductively harvested by a suitable antenna ofthe body-mountable device. Additionally or alternatively, power can begenerated from incident light harvested by photovoltaic cells (e.g.,solar cells). A rectifier and/or regulator can then output a stable DCvoltage to power the body-mountable device. Such an antenna can bearranged as a loop of conductive material that is connected toelectronics of the body-mountable device. Furthermore, thebody-mountable device can be configured to wirelessly communicateinformation (e.g., a signal related to a digital output of thesigma-delta modulator) to an external system by modifying the impedanceof the antenna so as to characteristically modify RF backscatter fromthe antenna in a manner that can be detected by the external system.

In some embodiments, the body-mountable device is situated in aneye-mountable device configured to rest on corneal tissue, similar to acontact lens. The body-mountable device includes a photodiode andelectronics configured to operate the photodiode (e.g., feedbackamplifiers, current mirrors, the sigma-delta modulator) and to performother functions of the body-mountable device (e.g., to wirelesslyindicate a signal related to the output of the sigma-delta modulator, topower the body-mountable device using radio frequency energy receivedusing an antenna) which can be disposed on a substrate embedded in thelens material. The substrate can be embedded near the periphery of theeye-mountable device, such as a ring-shaped substrate embedded in thecontact lens material around the circumference, so as to avoidinterference with light transmission to the pupil near the centralportion of the contact lens. The photodiode can be arranged on thesubstrate to face inward, toward the surface of the cornea, such thatthe photodiode can receive light from a portion of subsurfacevasculature of the eye (e.g., subsurface vasculature of the sclera). Thephotodiode may additionally or alternatively be arranged to faceoutward, away from the surface of the cornea, such that the sensorphotodiode can receive light from a portion of subsurface vasculature ofan eyelid at least partially covering the eye to which the eye-mountabledevice is mounted.

The body-mountable device could be operated in a variety of waysrelative to the digital output of the sigma-delta modulator. In someembodiments, a controller or other element(s) of the body-mountabledevice could be configured to determine a property of the blood in theportion of subsurface vasculature (e.g., a volume of blood, an oxygensaturation of blood) based on the digital output of the sigma-deltamodulator and/or other elements of the body-mountable device.Additionally or alternatively, the body-mountable device could beconfigured to indicate (e.g., wirelessly) to an external system a signalrelated to the digital output of the sigma-delta modulator, and theexternal system could determine a property of the blood in the portionof subsurface vasculature based on the indicated signal. The digitaloutput of the sigma-delta modulator could be converted (e.g., decimated)to generate a digital value related to the current through thephotodiode, and a property of the blood in the subsurface vasculaturecould be determined based on the generated digital value. Additionallyor alternatively, other operations could be performed relative to thedigital output of the sigma-delta modulator (e.g., a cross-correlationof the output could be performed to determine a pulse rate of blood inthe portion of subsurface vasculature). Conversions, decimation,determinations, or other operations or calculations related to thedigital output of the sigma-delta modulator could be performed by thebody-mountable device and/or by an external system receiving indicatedsignals from the body-mountable device related to the digital output ofthe sigma-delta modulator.

In some examples, an external reader can radiate radio frequencyradiation to power the body-mountable device via an energy harvestingsystem. The external reader may thereby control the operation of thebody-mountable device by controlling the supply of power to thebody-mountable device. In some examples, the external reader can operateto intermittently interrogate the body-mountable device to provide areading by radiating sufficient radiation to power the body-mountabledevice to obtain a measurement and communicate the result. The externalreader can also store the sensor results communicated by thebody-mountable device. In this way, the external reader can acquire aseries of received light measurements or other information related tolight received from a portion of subsurface vasculature over timewithout continuously powering the body-mountable device.

The external reader may be provided as a mobile device with softwareapplications for displaying the sensor results. The external reader mayalso include a communications interface that can be configured to conveymeasured and/or determined information to other systems for display,data storage, and/or analysis.

II. Example Ophthalmic Electronics Platform

FIG. 1 is a block diagram of a system 100 that includes an eye-mountabledevice 110 in wireless communication with an external reader 180. Theexposed regions of the eye-mountable device 110 are made of a polymericmaterial 120 formed to be contact-mounted to a corneal surface of aneye. A substrate 130 is embedded in the polymeric material 120 toprovide a mounting surface for a power supply 140, a controller 150,bio-interactive electronics 160, and a communication antenna 170. Thebio-interactive electronics 160 are operated by the controller 150. Thepower supply 140 supplies operating voltages to the controller 150and/or the bio-interactive electronics 160. The antenna 170 is operatedby the controller 150 to communicate information to and/or from theeye-mountable device 110. The antenna 170, the controller 150, the powersupply 140, and the bio-interactive electronics 160 can all be situatedon the embedded substrate 130. Because the eye-mountable device 110includes electronics and is configured to be contact-mounted to an eye,it is also referred to herein as an ophthalmic electronics platform.

To facilitate contact-mounting, the polymeric material 120 can have aconcave surface configured to adhere (“mount”) to a moistened cornealsurface (e.g., by capillary forces with a tear film coating the cornealsurface). Additionally or alternatively, the eye-mountable device 110can be adhered by a vacuum force between the corneal surface and thepolymeric material due to the concave curvature. While mounted with theconcave surface against the eye, the outward-facing surface of thepolymeric material 120 can have a convex curvature that is formed to notinterfere with eye-lid motion while the eye-mountable device 110 ismounted to the eye. For example, the polymeric material 120 can be asubstantially transparent curved polymeric disk shaped similarly to acontact lens.

The polymeric material 120 can include one or more biocompatiblematerials, such as those employed for use in contact lenses or otherophthalmic applications involving direct contact with the cornealsurface. The polymeric material 120 can optionally be formed in partfrom such biocompatible materials or can include an outer coating withsuch biocompatible materials. The polymeric material 120 can includematerials configured to moisturize the corneal surface, such ashydrogels and the like. In some instances, the polymeric material 120can be a deformable (“non-rigid”) material to enhance wearer comfort. Insome instances, the polymeric material 120 can be shaped to provide apredetermined, vision-correcting optical power, such as can be providedby a contact lens.

The substrate 130 includes one or more surfaces suitable for mountingthe bio-interactive electronics 160, the controller 150, the powersupply 140, and the antenna 170. The substrate 130 can be employed bothas a mounting platform for chip-based circuitry (e.g., by flip-chipmounting) and/or as a platform for patterning conductive materials(e.g., gold, platinum, palladium, titanium, copper, aluminum, silver,metals, other conductive materials, combinations of these, etc.) tocreate electrodes, interconnects, antennae, etc. In some embodiments,substantially transparent conductive materials (e.g., indium tin oxide)can be patterned on the substrate 130 to form circuitry, electrodes,etc. For example, the antenna 170 can be formed by depositing a patternof gold or another conductive material on the substrate 130. Similarly,interconnects 151, 157 between the controller 150 and thebio-interactive electronics 160, and between the controller 150 and theantenna 170, respectively, can be formed by depositing suitable patternsof conductive materials on the substrate 130. A combination ofmicrofabrication techniques including, without limitation, the use ofphotoresists, masks, deposition techniques and/or plating techniques canbe employed to pattern materials on the substrate 130. The substrate 130can be a relatively rigid material, such as polyethylene terephthalate(“PET”), parylene, or another material sufficient to structurallysupport the circuitry and/or electronics within the polymeric material120. The eye-mountable device 110 can alternatively be arranged with agroup of unconnected substrates rather than a single substrate. Forexample, the controller 150 and a bio-sensor or other bio-interactiveelectronic component can be mounted to one substrate, while the antenna170 is mounted to another substrate and the two can be electricallyconnected via the interconnects 157.

In some embodiments, the bio-interactive electronics 160 (and thesubstrate 130) can be positioned away from the center of theeye-mountable device 110 and thereby avoid interference with lighttransmission to the eye through the center of the eye-mountable device110. For example, where the eye-mountable device 110 is shaped as aconcave-curved disk, the substrate 130 can be embedded around theperiphery (e.g., near the outer circumference) of the disk. In someembodiments, the bio-interactive electronics 160 (and the substrate 130)can be positioned in the center region of the eye-mountable device 110.The bio-interactive electronics 160 and/or substrate 130 can besubstantially transparent to incoming visible light to mitigateinterference with light transmission to the eye. Moreover, in someembodiments, the bio-interactive electronics 160 can include a pixelarray 164 that emits and/or transmits light to be perceived by the eyeaccording to display instructions. Thus, the bio-interactive electronics160 can optionally be positioned in the center of the eye-mountabledevice so as to generate perceivable visual cues to a wearer of theeye-mountable device 110, such as by displaying information via thepixel array 164.

The substrate 130 can be shaped as a flattened ring with a radial widthdimension sufficient to provide a mounting platform for the embeddedelectronics components. The substrate 130 can have a thicknesssufficiently small to allow the substrate 130 to be embedded in thepolymeric material 120 without influencing the profile of theeye-mountable device 110. The substrate 130 can have a thicknesssufficiently large to provide structural stability suitable forsupporting the electronics mounted thereon. For example, the substrate130 can be shaped as a ring with a diameter of about 10 millimeters, aradial width of about 1 millimeter (e.g., an outer radius 1 millimeterlarger than an inner radius), and a thickness of about 50 micrometers.The substrate 130 can optionally be aligned with the curvature of theeye-mounting surface of the eye-mountable device 110 (e.g., convexsurface). For example, the substrate 130 can be shaped along the surfaceof an imaginary cone between two circular segments that define an innerradius and an outer radius. In such an example, the surface of thesubstrate 130 along the surface of the imaginary cone defines aninclined surface that is approximately aligned with the curvature of theeye mounting surface at that radius.

The power supply 140 is configured to harvest energy to power thecontroller 150 and bio-interactive electronics 160. For example, aradio-frequency energy-harvesting antenna 142 can capture energy fromincident radio radiation. Additionally or alternatively, solar cell(s)144 (“photovoltaic cells”) can capture energy from incoming ultraviolet,visible, and/or infrared radiation. Furthermore, an inertial powerscavenging system can be included to capture energy from ambientvibrations. The energy harvesting antenna 142 can optionally be adual-purpose antenna that is also used to communicate information to theexternal reader 180. That is, the functions of the communication antenna170 and the energy harvesting antenna 142 can be accomplished with thesame physical antenna.

A rectifier/regulator 146 can be used to condition the captured energyto a stable DC supply voltage 141 that is supplied to the controller150. For example, the energy harvesting antenna 142 can receive incidentradio frequency radiation. Varying electrical signals on the leads ofthe antenna 142 are output to the rectifier/regulator 146. Therectifier/regulator 146 rectifies the varying electrical signals to a DCvoltage and regulates the rectified DC voltage to a level suitable foroperating the controller 150. Additionally or alternatively, outputvoltage from the solar cell(s) 144 can be regulated to a level suitablefor operating the controller 150. The rectifier/regulator 146 caninclude one or more energy storage devices to mitigate high frequencyvariations in the ambient energy gathering antenna 142 and/or solarcell(s) 144. For example, one or more energy storage devices (e.g., acapacitor, an inductor, etc.) can be connected in parallel across theoutputs of the rectifier 146 to regulate the DC supply voltage 141 andconfigured to function as a low-pass filter.

The controller 150 is turned on when the DC supply voltage 141 isprovided to the controller 150, and the logic in the controller 150operates the bio-interactive electronics 160 and the antenna 170. Thecontroller 150 can include logic circuitry configured to operate thebio-interactive electronics 160 so as to interact with a biologicalenvironment of the eye-mountable device 110. The interaction couldinvolve the use of one or more components, such as a photodiode 162, inbio-interactive electronics 160 to obtain input from the biologicalenvironment (e.g., from a portion of subsurface vasculature in thebiological environment). Additionally or alternatively, the interactioncould involve the use of one or more components, such as pixel array164, to provide an output to the biological environment.

In one example, the controller 150 includes a sensor interface module152 that is configured to operate photodiode 162. The photodiode 162 canbe operated to generate an output related to light (e.g., visible light,infrared light, ultraviolet light) received from the biologicalenvironment. For example, the sensor interface 152 could include aphotodiode voltage source configured to apply a specified voltage to thephotodiode 162 such that the photodiode 162 is reverse biased and suchthat a current through the photodiode 162 is related to the lightreceived from the biological environment by the photodiode 162. Forexample, the current through the photodiode 162 could be related to theintensity of the received light, the intensity of the received lightwithin a specified range of wavelengths, the polarization of the light,or some other property of the received light. The sensor interface 152could further include a current mirror and a sigma-delta modulatorconfigured such that the current mirror creates an output currentrelated to the current through the photodetector 162 and presents theoutput current to the sigma-delta modulator. The sigma-delta modulatorcould be configured to produce a digital output that is related to theoutput current. The digital output could be used for a variety ofapplications. Additionally or alternatively, one or more electronicelements or systems configured to operate the photodiode 162 could bedisposed as part of the bio-interactive electronics 160 (e.g., by beingformed from the same integrated circuit or semiconductor wafer as thephotodiode 162) or as part of some other aspect of the eye-mountabledevice 110.

The light received by the photodiode 162 could be received from thebiological environment in response to illumination. The illuminationcould be illumination from ambient light sources (e.g., the sun, a lamp,some other artificial light source, light reflected off of objectstoward the biological environment). The photodiode 162 could beconfigured to receive light from a particular angle or location relativeto the biological environment. For example, the photodiode 162 could beconfigured to receive light from a portion of subsurface vasculature ofan eye by being disposed on and/or configured to receive light through aconcave surface of the eye-mountable device 110 (i.e., by being directedtoward the concave surface). In some examples, the eye-mountable device110 could be configured (e.g., weighted, formed with a specified shape)such that the photodiode 162 received light from subsurface vasculaturedisposed in a lateral region of an eye relative to a pupil of the eye.Additionally or alternatively, the photodiode 162 could be configured toreceive light from a portion of subsurface vasculature of an eyelid ofan eye when mounted to the eye, by being disposed on and/or configuredto receive light through a convex surface of the eye-mountable device110 (i.e., by being directed toward the convex surface). Theeye-mountable device 110 could include multiple photodiodes configuredto detect light from multiple portions of a biological environment(e.g., multiple portions of subsurface vasculature of an eye/eyelid)and/or from other sources (e.g., to detect a level of ambient light inthe environment of the eye-mountable device 110). The eye-mountabledevice 110 could include filters, mirrors, lenses, diffraction gratings,or other optical elements configured to focus, block, or otherwisemodify light from the biological environment that is received by thephotodiode 162.

The controller 150 can optionally include a display driver module 154for operating a pixel array 164. The pixel array 164 can be an array ofseparately programmable light transmitting, light reflecting, and/orlight emitting pixels arranged in rows and columns. The individual pixelcircuits can optionally include liquid crystal technologies,microelectromechanical technologies, emissive diode technologies, etc.to selectively transmit, reflect, and/or emit light according toinformation from the display driver module 154. Such a pixel array 164can also optionally include more than one color of pixels (e.g., red,green, and blue pixels) to render visual content in color. The displaydriver module 154 can include, for example, one or more data linesproviding programming information to the separately programmed pixels inthe pixel array 164 and one or more addressing lines for setting groupsof pixels to receive such programming information. Such a pixel array164 situated on the eye can also include one or more lenses to directlight from the pixel array to a focal plane perceivable by the eye.

The controller 150 can also include a communication circuit 156 forsending and/or receiving information via the antenna 170. Thecommunication circuit 156 can optionally include one or moreoscillators, mixers, frequency injectors, etc. to modulate and/ordemodulate information on a carrier frequency to be transmitted and/orreceived by the antenna 170. In some examples, the eye-mountable device110 is configured to indicate an output from a bio-sensor by modulatingan impedance of the antenna 170 in a manner that is perceivably by theexternal reader 180. For example, the communication circuit 156 cancause variations in the amplitude, phase, and/or frequency ofbackscatter radiation from the antenna 170, and such variations can bedetected by the reader 180.

The controller 150 is connected to the bio-interactive electronics 160via interconnects 151. For example, where the controller 150 includeslogic elements implemented in an integrated circuit to form the sensorinterface module 152 and/or display driver module 154, a patternedconductive material (e.g., gold, platinum, palladium, titanium, copper,aluminum, silver, metals, combinations of these, etc.) can connect aterminal on the chip to the bio-interactive electronics 160. Similarly,the controller 150 is connected to the antenna 170 via interconnects157.

It is noted that the block diagram shown in FIG. 1 is described inconnection with functional modules for convenience in description.However, embodiments of the eye-mountable device 110 can be arrangedwith one or more of the functional modules (“sub-systems”) implementedin a single chip, integrated circuit, and/or physical feature. Forexample, while the rectifier/regulator 146 is illustrated in the powersupply block 140, the rectifier/regulator 146 can be implemented in achip that also includes the logic elements of the controller 150 and/orother features of the embedded electronics in the eye-mountable device110. Thus, the DC supply voltage 141 that is provided to the controller150 from the power supply 140 can be a supply voltage that is providedon a chip by rectifier and/or regulator components the same chip. Thatis, the functional blocks in FIG. 1 shown as the power supply block 140and controller block 150 need not be implemented as separated modules.Moreover, one or more of the functional modules described in FIG. 1 canbe implemented by separately packaged chips electrically connected toone another.

Additionally or alternatively, the energy harvesting antenna 142 and thecommunication antenna 170 can be implemented with the same physicalantenna. For example, a loop antenna can both harvest incident radiationfor power generation and communicate information via backscatterradiation.

The external reader 180 includes an antenna 188 (or group of more thanone antenna) to send and receive wireless signals 171 to and from theeye-mountable device 110. The external reader 180 also includes acomputing system with a processor 186 in communication with a memory182. The external reader 180 can also include one or more of usercontrols 185, a display 187, and a communication interface 189. Thememory 182 is a non-transitory computer-readable medium that caninclude, without limitation, magnetic disks, optical disks, organicmemory, and/or any other volatile (e.g. RAM) or non-volatile (e.g. ROM)storage system readable by the processor 186. The memory 182 can includea data storage 183 to store indications of data, such as sensor readings(e.g., related to the photodiode 162), program settings (e.g., to adjustbehavior of the eye-mountable device 110 and/or external reader 180),etc. The memory 182 can also include program instructions 184 forexecution by the processor 186 to cause the external reader 180 toperform processes specified by the instructions 184. For example, theprogram instructions 184 can cause external reader 180 to perform any ofthe function described herein. For example, program instructions 184 maycause the external reader 180 to provide a user interface that allowsfor retrieving information communicated from the eye-mountable device110 (e.g., sensor outputs or other information related to the photodiode162) by displaying that information on the display 187 in response tocommands input through the user controls 185. The external reader 180can also include one or more hardware components for operating theantenna 188 to send and receive the wireless signals 171 to and from theeye-mountable device 110. For example, oscillators, frequency injectors,encoders, decoders, amplifiers, filters, etc. can drive the antenna 188according to instructions from the processor 186.

The external reader 180 can also be configured include a communicationinterface 189 to communicate signals via a communication medium 191 toand from a remote system 190. For example, the remote system 190 may bea smart phone, tablet computer, laptop computer, or personal computer,and communication interface 189 and communication medium 191 may be aBluetooth module and wireless Bluetooth communication signals,respectively. In this example, the external reader 180 may be configuredto send ion concentration data collected by the biosensor 160 to thesmart phone, tablet computer, laptop computer, or personal computer forstorage and offline analysis. In another example, the remote system 190is a server at a clinic or physician's office, the communicationinterface 189 is a WiFi radio module, and the communication medium 191is elements of the internet sufficient to enable the transfer of databetween the remote server and the WiFi radio module. A physician may usethis data to make determinations or diagnoses related to the subject'scondition. Further, the external reader 180 may be configured to receivesignals from a remote server, such as instructions sent by a physicianat a remote location to, for example, increase or decrease samplingfrequency. Communication interface 189 could be configured to enableother forms of wired or wireless communication; for example, CDMA, EVDO,GSM/GPRS, WiMAX, LTE, infrared, ZigBee, Ethernet, USB, FireWire, a wiredserial link, or near field communication.

The external reader 180 can be a smart phone, digital assistant, orother portable computing device with wireless connectivity sufficient toprovide the wireless communication link 171. The external reader 180 canalso be implemented as an antenna module that can be plugged into aportable computing device, such as in an example where the communicationlink 171 operates at carrier frequencies not commonly employed inportable computing devices. In some instances, the external reader 180is a special-purpose device configured to be worn relatively near awearer's eye to allow the wireless communication link 171 to operatewith a low power budget. For example, the external reader 180 can beintegrated in a piece of jewelry such as a necklace, earring, etc. orintegrated in an article of clothing worn near the head, such as a hat,headband, etc. The external reader 180 could also be implemented in eyeglasses or a head-mounted display.

In an example where the eye-mountable device 110 includes a photodiode162 and related electronics as described herein, the system 100 can beoperated to measure one or more properties of a biological tissue (e.g.,of blood within or other aspects of a portions of subsurfacevasculature) by receiving light (e.g., visible light, infrared light,ultraviolet light) from the biological tissue. For example, lightreceived from a portion of subsurface vasculature could be related to(e.g., could have an intensity related to) a volume of blood in theportion of subsurface vasculature (e.g., by being related to a degree ofabsorption of ambient light by the blood). One or more properties of thereceived light detected and/or determined using the photodiode 162 andrelated electronics could be used to determine a volume of blood in theportion of subsurface vasculature and/or other information related tosuch. For example, the intensity of the received light could bedetermined a plurality of times during a plurality of periods of time,and a pulse rate, a blood flow rate, a blood pressure, a blood flow rateand/or pressure profile, or some other information about the blood inthe portion of subsurface vasculature could be determined based on theplurality of determined intensities of the received light.

In another example, the eye-mountable device 110 includes a photodiode162 and related electronics as described herein. The eye-mountabledevice 110 further includes a photodetector. The photodetector couldinclude a photodiode and related electronics configured similarly to thephotodiode 162 and related electronics, could share one or morecomponents (e.g., a common sigma-delta modulator) with the photodiode162 and related electronics, or could be configured in some other way.Further, the photodiode 162 and photodetector are configured to detectan intensity of light received from a portion of subsurface vasculaturewithin respective first and second ranges of wavelengths. An oxygensaturation of blood in the portion of subsurface vasculature could bedetermined based on the intensities detected by the photodiode 162 andthe photodetector. Other properties of a biological environment could bedetected and/or determined using the eye-mountable device 110 (e.g.,using the photodiode 162 and related electronics and/or other componentsof the eye-mountable device 110).

To perform a reading with the system 100 configured as a light sensor,the external reader 180 can emit radio frequency radiation 171 that isharvested to power the eye-mountable device 110 via the power supply140. Radio frequency electrical signals captured by the energyharvesting antenna 142 (and/or the communication antenna 170) arerectified and/or regulated in the rectifier/regulator 146 and aregulated DC supply voltage 147 is provided to the controller 150. Theradio frequency radiation 171 thus powers the electronic componentswithin the eye-mountable device 110. Once powered, the controller 150operates the photodiode 162 to measure one or more properties of lightreceived from a biological environment at one or more points in time.For example, the sensor interface module 152 can generate a digitaloutput (e.g., using a sigma-delta modulator and other electronics of thesensor interface module) related to the light received by the photodiode162 (e.g., to an intensity of the received light and/or to an intensityof the received light within one or more specified ranges ofwavelengths). The measured property or properties can provide the sensorreading (“result”) indicative of one or more properties of thebiological environment (e.g., an absolute or relative volume of bloodwithin a portion of subsurface vasculature at one or more points intime, an extinction coefficient of the blood in a first range ofwavelengths relative to an extinction coefficient of the blood in asecond range of wavelengths).

The controller 150 can operate the antenna 170 to communicate the sensorreading back to the external reader 180 (e.g., via the communicationcircuit 156). Additionally or alternatively, the controller 150 canoperate the antenna 170 to communicate other information. For example,the controller 150 could be configured to determine one or moreproperties of the biological environment based on one or more sensorreadings (e.g., could determine an oxygen saturation of blood based onrelative intensities of light received from the blood within respectiveranges wavelengths) and to subsequently communicate the determined oneor more properties back to the external reader 180 using the antenna170. In another example, the digital output of the sigma-delta modulator(i.e., a series of digital pulses having respective durations duringrespective periods of time) could be indicated or otherwise communicatedto the external reader 180 using the antenna 170. The sensor reading orother information can be communicated by, for example, modulating animpedance of the communication antenna 170 such that the modulation inimpedance is detected by the external reader 180. The modulation inantenna impedance can be detected by, for example, backscatter radiationfrom the antenna 170.

In some embodiments, the system 100 can operate to non-continuously(“intermittently”) supply energy to the eye-mountable device 110 topower the controller 150 and electronics 160. For example, radiofrequency radiation 171 can be supplied to power the eye-mountabledevice 110 long enough to carry out a light measurement and communicatethe results. For example, the supplied radio frequency radiation canprovide sufficient power to measure the light received from thebiological environment by the photodiode 162 and modulate the antennaimpedance to adjust the backscatter radiation in a manner indicative ofthe measured light. In such an example, the supplied radio frequencyradiation 171 can be considered an interrogation signal from theexternal reader 180 to the eye-mountable device 110 to request ameasurement. By periodically interrogating the eye-mountable device 110(e.g., by supplying radio frequency radiation 171 to temporarily turnthe device on) and storing the sensor results (e.g., via the datastorage 183), the external reader 180 can accumulate a set of light orother measurements and/or related determined properties (e.g., bloodvolumes, blood oxygen saturations, pulse rates) over time withoutcontinuously powering the eye-mountable device 110.

In other embodiments, the system 100 can operate continuously and supplyenergy to the eye-mountable device 110 to power the controller 150 andelectronics 160 at all times. In some instances, it may be desirable tocontinuously measure light received from a biological environment inresponse to illumination (e.g., ambient illumination) and/or to performsome determination related to such a measurement and to collect, store,and or transmit this data.

FIG. 2A is a bottom view of an example eye-mountable electronic device210. FIG. 2B is an aspect view of the example eye-mountable electronicdevice shown in FIG. 2A. It is noted that relative dimensions in FIGS.2A and 2B are not necessarily to scale, but have been rendered forpurposes of explanation only in describing the arrangement of theexample eye-mountable electronic device 210. The eye-mountable device210 is formed of a polymeric material 220 shaped as a curved disk. Thepolymeric material 220 can be a substantially transparent material toallow incident light to be transmitted to the eye while theeye-mountable device 210 is mounted to the eye. The polymeric material220 can be a biocompatible material similar to those employed to formvision correction and/or cosmetic contact lenses in optometry, such aspolyethylene terephthalate (“PET”), polymethyl methacrylate (“PMMA”),silicone hydrogels, combinations of these, etc. The polymeric material220 can be formed with one side having a concave surface 226 suitable tofit over a corneal surface of an eye. The opposing side of the disk canhave a convex surface 224 that does not interfere with eyelid motionwhile the eye-mountable device 210 is mounted to the eye. A circularouter side edge 228 connects the concave surface 224 and convex surface226.

The eye-mountable device 210 can have dimensions similar to a visioncorrection and/or cosmetic contact lenses, such as a diameter ofapproximately 1 centimeter, and a thickness of about 0.1 to about 0.5millimeters. However, the diameter and thickness values are provided forexplanatory purposes only. In some embodiments, the dimensions of theeye-mountable device 210 can be selected according to the size and/orshape of the corneal surface of the wearer's eye.

The polymeric material 220 can be formed with a curved shape in avariety of ways. For example, techniques similar to those employed toform vision-correction contact lenses, such as heat molding, injectionmolding, spin casting, etc. can be employed to form the polymericmaterial 220. While the eye-mountable device 210 is mounted in an eye,the convex surface 224 faces outward to the ambient environment whilethe concave surface 226 faces inward, toward the corneal surface. Theconvex surface 224 can therefore be considered an outer, top surface ofthe eye-mountable device 210 whereas the concave surface 226 can beconsidered an inner, bottom surface. The “bottom” view shown in FIG. 2Ais facing the concave surface 226. From the bottom view shown in FIG.2A, the outer periphery 222, near the outer circumference of the curveddisk is curved out of the page, whereas the center region 221, near thecenter of the disk is curved in to the page.

A substrate 230 is embedded in the polymeric material 220. The substrate230 can be embedded to be situated along the outer periphery 222 of thepolymeric material 220, away from the center region 221. The substrate230 does not interfere with vision because it is too close to the eye tobe in focus and is positioned away from the center region 221 whereincident light is transmitted to the eye-sensing portions of the eye.Moreover, the substrate 230 can be formed of a transparent material tofurther mitigate any effects on visual perception.

The substrate 230 can be shaped as a flat, circular ring (e.g., a diskwith a central hole). The flat surface of the substrate 230 (e.g., alongthe radial width) is a platform for mounting electronics such as chips(e.g., via flip-chip mounting) and for patterning conductive materials(e.g., via deposition techniques) to form electrodes, antenna(e), and/orconnections. The substrate 230 and the polymeric material 220 can beapproximately cylindrically symmetric about a common central axis. Thesubstrate 230 can have, for example, a diameter of about 10 millimeters,a radial width of about 1 millimeter (e.g., an outer radius 1 millimetergreater than an inner radius), and a thickness of about 50 micrometers.However, these dimensions are provided for example purposes only, and inno way limit the present disclosure. The substrate 230 can beimplemented in a variety of different form factors.

A loop antenna 270, controller 250, and bio-interactive electronics 260are disposed on the embedded substrate 230. The controller 250 can be achip including logic elements configured to operate the bio-interactiveelectronics 260 and the loop antenna 270. The controller 250 iselectrically connected to the loop antenna 270 by interconnects 257 alsosituated on the substrate 230. Similarly, the controller 250 iselectrically connected to the bio-interactive electronics 260 by aninterconnect 251. The interconnects 251, 257, the loop antenna 270, andany conductive electrodes (e.g., for an electrochemical ion sensor,etc.) can be formed from conductive materials patterned on the substrate230 by a process for precisely patterning such materials, such asdeposition, lithography, etc. The conductive materials patterned on thesubstrate 230 can be, for example, gold, platinum, palladium, titanium,carbon, aluminum, copper, silver, silver-chloride, conductors formedfrom noble materials, metals, combinations of these, etc.

As shown in FIG. 2A, which is a view facing the concave surface 226 ofthe eye-mountable device 210, the bio-interactive electronics module 260is mounted to a side of the substrate 230 facing the concave surface226. Where the bio-interactive electronics module 260 includes a lightsensor (e.g., a photodiode), for example, mounting such a sensor on thesubstrate 230 to be close to the concave surface 226 allows the sensorto sense light (e.g., visible light, infrared light, ultraviolet light)received from portions of subsurface vasculature or other tissues nearthe surface of the eye. However, the electronics, electrodes, etc.situated on the substrate 230 can be mounted to either the “inward”facing side (e.g., situated closest to the concave surface 226) or the“outward” facing side (e.g., situated closest to the convex surface224). Moreover, in some embodiments, some electronic components can bemounted on one side of the substrate 230, while other electroniccomponents are mounted to the opposing side, and connections between thetwo can be made via conductive materials passing through the substrate230.

The loop antenna 270 can be a layer of conductive material patternedalong the flat surface of the substrate to form a flat conductive ring.In some instances, the loop antenna 270 can be formed without making acomplete loop. For instance, the antenna 270 can have a cutout to allowroom for the controller 250 and bio-interactive electronics 260, asillustrated in FIG. 2A. However, the loop antenna 270 can also bearranged as a continuous strip of conductive material that wrapsentirely around the flat surface of the substrate 230 one or more times.For example, a strip of conductive material with multiple windings canbe patterned on the side of the substrate 230 opposite the controller250 and bio-interactive electronics 260. Interconnects between the endsof such a wound antenna (e.g., the antenna leads) can be passed throughthe substrate 230 to the controller 250.

FIG. 2C is a side cross-section view of the example eye-mountableelectronic device 210 while mounted to a corneal surface 22 of an eye10. FIG. 2D is a close-in side cross-section view enhanced to show thetear film layers 40, 42 surrounding the exposed surfaces 224, 226 of theexample eye-mountable device 210. It is noted that relative dimensionsin FIGS. 2C and 2D are not necessarily to scale, but have been renderedfor purposes of explanation only in describing the arrangement of theexample eye-mountable electronic device 210. For example, the totalthickness of the eye-mountable device can be about 200 micrometers,while the thickness of the tear film layers 40, 42 can each be about 10micrometers, although this ratio may not be reflected in the drawings.Some aspects are exaggerated to allow for illustration and facilitateexplanation.

The eye 10 includes a cornea 20 that is covered by bringing the uppereyelid 30 and lower eyelid 32 together over the top of the eye 10.Incident light is received by the eye 10 through the cornea 20, wherelight is optically directed to light sensing elements of the eye 10(e.g., rods and cones, etc.) to stimulate visual perception. The motionof the eyelids 30, 32 distributes a tear film across the exposed cornealsurface 22 of the eye 10. The tear film is an aqueous solution secretedby the lacrimal gland to protect and lubricate the eye 10. When theeye-mountable device 210 is mounted in the eye 10, the tear film coatsboth the concave and convex surfaces 224, 226 with an inner layer 40(along the concave surface 226) and an outer layer 42 (along the convexlayer 224). The tear film layers 40, 42 can be about 10 micrometers inthickness and together account for about 10 microliters.

The tear film layers 40, 42 are distributed across the corneal surface22 and/or the convex surface 224 by motion of the eyelids 30, 32. Forexample, the eyelids 30, 32 raise and lower, respectively, to spread asmall volume of tear film across the corneal surface 22 and/or theconvex surface 224 of the eye-mountable device 210. The tear film layer40 on the corneal surface 22 also facilitates mounting the eye-mountabledevice 210 by capillary forces between the concave surface 226 and thecorneal surface 22. In some embodiments, the eye-mountable device 210can also be held over the eye in part by vacuum forces against cornealsurface 22 due to the concave curvature of the eye-facing concavesurface 226.

As shown in the cross-sectional views in FIGS. 2C and 2D, the substrate230 can be inclined such that the flat mounting surfaces of thesubstrate 230 are approximately parallel to the adjacent portion of theconcave surface 226. As described above, the substrate 230 is aflattened ring with an inward-facing surface 232 (closer to the concavesurface 226 of the polymeric material 220) and an outward-facing surface234 (closer to the convex surface 224). The substrate 230 can haveelectronic components and/or patterned conductive materials mounted toeither or both mounting surfaces 232, 234. As shown in FIG. 2D, thebio-interactive electronics 260, controller 250, and conductiveinterconnect 251 are mounted on the inward-facing surface 232 such thatthe bio-interactive electronics 260 are relatively closer in proximityto the inner tear film layer 42 and the cornea 20 and/or other elementsof the eye (e.g., sclera, iris, portions of subsurface vasculaturethereof) than if they were mounted on the outward-facing surface 234.With this arrangement, the bio-interactive electronics 260 can receivelight from elements of the eye (e.g., cornea 20, sclera, iris, portionsof subsurface vasculature thereof) through the concave surface 226.However, in other examples, the bio-interactive electronics 260 may bemounted on the outward-facing surface 234 of the substrate 230 such thatthe bio-interactive electronics 260 are facing the convex surface 224and able to receive light from an environment away from the eye 10(e.g., from an ambient or other light source in the environment of theeye 10, from a portion of subsurface vasculature of an eyelid 30, 32when said eyelid is wholly or partially covering the bio-interactiveelectronics of the eye-mountable device 210.

The bio-active electronics 260 include a photodiode and relatedelectronics configured to operate the photodiode. FIG. 2E illustrates afront view (i.e., a view from in front of the eye 10) of theeye-mountable device 210 when it is mounted to the eye 10. Thephotodiode of the bio-active electronics 260 is directed toward theconcave surface 226 such that the photodiode receives light from a firstportion of subsurface vasculature 25 disposed in a lateral region of theeye 10 relative to the pupil 23 of the eye 10 (e.g., beneath and/orwithin the sclera of the eye 10).

The photodiode and other elements of the bio-active electronics 260 canbe operated and/or configured to generate an output related to lightreceived from the first portion of subsurface vasculature 25. Forexample, the bio-active electronics 260 and/or the controller 250 (notshown in FIG. 2E) could include a photodiode voltage source configuredto apply a specified voltage to the photodiode such that the photodiodeis reverse biased and such that a current through the photodiode isrelated to the light received from the first portion of subsurfacevasculature 25 by the photodiode. For example, the current through thephotodiode could be related to the intensity of the received light, theintensity of the received light within a specified range of wavelengths,the polarization of the light, or some other property of the receivedlight. The bio-active electronics 260 and/or the controller 250 couldfurther include a current mirror and a sigma-delta modulator configuredsuch that the current mirror creates an output current related to thecurrent through the photodetector and presents the output current to thesigma-delta modulator. The sigma-delta modulator could be configured toproduce a digital output that is related to the output current. Thedigital output could be used for a variety of applications.

The eye-mountable device could include additional or alternativephotodiodes and/or other elements of bio-active electronics disposed indifferent locations relative to the eye-mountable device 210 and/orelements of the eye 10 and directed in different directions such thatthe additional or alternative photodiodes receive light from othertissues of the eye and/or from the environment of the eye. FIG. 2Fillustrates a front view (i.e., a view from in front of the eye 10) ofthe eye-mountable device 210 when it is mounted to the eye 10 and theupper eyelid 30 is lowered partially over the eye-mountable device 210.FIG. 2F illustrates additional bio-active electronics 263 configuredsimilarly to the bio-active electronics 260 (i.e., including aphotodiode and related electronics configured to operate the photodiodeto receive light from an environment). The photodiode of the additionalbio-active electronics 263 is directed toward the convex surface 224such that the photodiode receives light from a second portion ofsubsurface vasculature 35 disposed in the upper eyelid 30 of the eye 10.

Related electronics configured to operate the photodiode of theadditional bio-active electronics 263 could be disposed in one or bothof the controller 250 (not shown in FIG. 2F) and the additionalbio-active electronics 263 and could be configured similarly to theelectronics described elsewhere herein regarding operation of aphotodiode to detect light received from an environment. Further, one ormore components or subsystems (e.g., a sigma-delta modulator) could beshared between the bio-active electronics 260 and the additionalbio-active electronics 263 (e.g., by including electronic switchesconfigured to multiplex the input of the sigma-delta modulator from thecurrent mirrors and/or photodiode voltage sources corresponding to thephotodiodes of the bio-active electronics 260 and the additionalbio-active electronics 263).

Note that the locations of the first 25 and second 35 portions ofsubsurface vasculature and corresponding locations of the bio-activeelectronics 260 and the additional bio-active electronics 263 relativeto the eye-mountable device 210, the eye 10, and/or the portions ofsubsurface vasculature 25, 35 are intended as non-limiting, illustrativeexamples. Bio-active electronics, photodiodes, or other elements of suchan eye-mountable device could have different locations than thoseillustrated. Further, the location of an element of an eye-mountabledevice (e.g., of the bio-active electronics 260) relative to elements ofthe eye 10, eyelids 30, 32, portions of subsurface vasculature 25, 35 orother elements could be controlled by a number of methods. For example,the eye-mountable device 210 could be weighted such that the orientationof the eye-mountable device 210 relative to the direction of gravity(and by extension, relative to elements of the eye 10) is controlled.Additionally or alternatively, the eye-mountable device 210 could formedor shaped relative to a shape of the cornea 20, sclera, eyelids 30, 32,or other elements of the eye such that the orientation of theeye-mountable device 210 relative to such elements of the eye 10 iscontrolled. For example, a lower edge of the polymeric material 220could be flattened, such that movement of the lower eyelid 32 againstthe flat lower edge of the polymeric material 220 could act to orientthe eye-mountable device 210 relative to the lower eyelid 32.

In some examples, the eye-mountable device 210 could include a pluralityof photodiodes arranged on or within the eye-mountable device 210according to an application. For example, a plurality of photodiodescould be arranged in a curved linear array wholly or partiallyencircling the central region of the eye-mountable device 210.Photodiodes of such an array could be operated as described herein(e.g., by photodiode voltage sources, current mirrors, and sigma-deltamodulators) to detect light received from respective portions ofsubsurface vasculature of the eye or from some other environment toallow some application as described herein (e.g., detection of bloodflow, pulse rate, blood oxygen saturation). In such examples, theplurality of photodiodes could be operated to determine the location ofa target portion of vasculature. An individual photodiode of the arraythat is proximate to the target portion of subsurface vasculature couldthen be used to determine one or more properties of the portion ofvasculature (e.g., blood flow, pulse rate) as described herein. Such aplurality of photodiodes (e.g., a curved linear array of photodiodes)could be operated to allow additional applications; for example, apattern of vasculature or other property of an eye and/or eyelid couldbe detected and used, e.g., to identify a wearer of the eye-mountabledevice 210.

Moreover, it is particularly noted that while the body-mountable lightsensor platform described herein by way of example as an eye-mountabledevice or an ophthalmic device, it is noted that the disclosed lightsensing photodiodes and related low-power electronics (e.g., currentmirror, photodiode voltage source, sigma-delta modulator) can be appliedin other contexts as well. For example, light sensors disclosed hereinmay be included in wearable (e.g., body-mountable) and/or implantablelight sensors. In some contexts, photodiode and related electronics issituated to be substantially encapsulated by bio-compatible polymericmaterial suitable for being in contact with an external body surfaceand/or for being implanted such that light can be received from aportion of subsurface vasculature related, e.g., to the volume of bloodin the portion of subsurface vasculature. In one example, amouth-mountable device includes a photodiode and related electronics andis configured to be mounted within an oral environment, such as adjacenta tooth or adhered to an inner mouth surface. In another example, animplantable medical device that includes a photodiode and relatedelectronics may be encapsulated in biocompatible material and implantedwithin a host organism. Such body-mounted and/or implanted light sensorscan include circuitry configured to operate a photodiode and relatedelectronics by providing power to the electronics, by decimating orperforming some other operation on an output of the electronics, or someother function(s). The light sensor can also include an energyharvesting system and a communication system for wirelessly indicatingthe sensor results (i.e., received light).

In other examples, light sensors disclosed herein may be included inwireless light sensors which are not used to measure light received froma portion of subsurface or other vasculature in a human body. Forexample, light sensors (i.e., photodiodes and related electronics)disclosed herein may be included in body-mountable and/or implantablelight sensors used to measure light received from a portion ofsubsurface or other vasculature of an animal. In another example, lightsensors disclosed herein may be included in devices to measure lightreceived from a portion of some other environment in response toillumination (e.g., by one or more ambient light sources), such as afluid or other element(s) of a river, lake, marsh, forest, prairie,reservoir, water supply, sanitary sewer system, or storm sewer system.For example, light sensors disclosed herein could be included in alow-power environmental sensor that is part of a distributed sensornetwork. Other applications for photodiodes and related electronics asdescribed herein (e.g., to perform a high-resolution, low-powermeasurement of light received from a target environment) areanticipated.

III. A Body-Mountable Light Sensor

FIG. 3 is a functional block diagram of a system 300 for measuring light(e.g., visible light, infrared light, ultraviolet light) received from abiological environment (e.g., from a portion of subsurface vasculature,e.g., of a sclera or eyelid of an eye). The system 300 includes abody-mountable device 310 with embedded electronic components powered byan external reader 340. The body-mountable device 310 includes anantenna 312 for capturing radio frequency radiation 341 from theexternal reader 340. The body-mountable device 310 includes a rectifier314, an energy storage element 316, and a regulator 318 for generatingpower supply voltages 330, 332 to operate the embedded electronics. Thebody-mountable device 310 includes a photodiode 322 operated by a sensorinterface 321. The body-mountable device 310 includes hardware logic 324for communicating outputs and/or information derived therefrom from thesensor interface 321 to the external reader 340 by modulating (by meansof modulation electronics and interconnects 325) the impedance of theantenna 312. Similar to the eye-mountable devices 110, 210 discussedabove in connection with FIGS. 1 and 2A-2F, the body-mountable device310 can include a mounting substrate embedded within a polymericmaterial configured to be mounted to an eye. The photodiode 322 can besituated on a mounting surface of such a substrate proximate to thesurface of the eye (e.g., corresponding to the bio-interactiveelectronics 260 on the inward-facing side 232 of the substrate 230) tomeasure light received from tissues of the eye (e.g., cornea 20, sclera,iris, and/or portions of subsurface vasculature thereof). Alternatively,the photodiode 322 can be situated on a mounting surface of such asubstrate distal to the surface of the eye (e.g., on the outward-facingside 234 of the substrate 230) to measure light received from tissues ofthe eyelid (e.g., portions of subsurface vasculature of the eyelid)and/or from an environment of the eye.

The sensor interface 321 is configured to operate the photodiode 322 togenerate an output related to light received from the biologicalenvironment. For example, the sensor interface 321 could include aphotodiode voltage source configured to apply a specified voltage to thephotodiode 322 such that the photodiode 322 is reverse biased and suchthat a current through the photodiode 322 is related to the lightreceived from the biological environment by the photodiode 322. Forexample, the current through the photodiode 322 could be related to theintensity of the received light, the intensity of the received lightwithin a specified range of wavelengths, the polarization of the light,or some other property of the received light. The sensor interface 321could further include a current mirror and a sigma-delta modulatorconfigured such that the current mirror creates an output currentrelated to the current through the photodetector 322 and presents theoutput current to the sigma-delta modulator. The sigma-delta modulatorcould be configured to produce a digital output that is related to theoutput current. The digital output could be used for a variety ofapplications.

The rectifier 314, energy storage 316, and voltage regulator 318 operateto harvest energy from received radio frequency radiation 341. The radiofrequency radiation 341 causes radio frequency electrical signals onleads of the antenna 312. The rectifier 314 is connected to the antennaleads and converts the radio frequency electrical signals to a DCvoltage. The energy storage element 316 (e.g., capacitor) is connectedacross the output of the rectifier 314 to filter high frequency noise onthe DC voltage. The regulator 318 receives the filtered DC voltage andoutputs both a digital supply voltage 330 to operate the hardware logic324 and an analog supply voltage 332 to operate the sensor interface 321and photodiode 322. For example, the analog supply voltage can be avoltage used by the sensor interface 321 to reverse bias thephotodetector 322 and to generate a digital output related to lightreceived by the photodiode 322. The digital supply voltage 330 can be avoltage suitable for driving digital logic circuitry, such asapproximately 1.2 volts, approximately 3 volts, etc. Reception of theradio frequency radiation 341 from the external reader 340 (or anothersource, such as ambient radiation, etc.) causes the supply voltages 330,332 to be supplied to the sensor interface 321 and hardware logic 324.While powered, the sensor interface 321 and hardware logic 324 areconfigured to measure light received by the photodiode 322 andcommunicate the results.

The sensor results (i.e., an output of the sensor interface 321 and/orother signals or information derived therefrom) can be communicated backto the external reader 340 via backscatter radiation 343 from theantenna 312. The hardware logic 324 receives the digital output from thesensor interface 321 and modulates (325) the impedance of the antenna312 in accordance with the digital output that is related to the lightreceived by the photodiode 322. The antenna impedance and/or change inantenna impedance is detected by the external reader 340 via thebackscatter signal 343. The external reader 340 can include an antennafront end 342 and logic components 344 to decode the informationindicated by the backscatter signal 343 and provide digital inputs to aprocessing system 346. The external reader 340 associates thebackscatter signal 343 with the sensor result (e.g., via the processingsystem 346 according to a pre-programmed relationship associatingpatterns over time (e.g., pulse durations and timings) and/or levels ofimpedance of the antenna 312 with properties of the light received bythe photodiode 322). The processing system 346 can then store theindicated sensor results (e.g., received light intensities within one ormore ranges of wavelengths, blood volumes, blood flow rates, pulserates, blood oxygenation, timings of heart beats) in a local memoryand/or a network-connected memory. Alternatively, the sensor results canbe communicated back to the external reader 340 via an internallygenerated radio frequency signal 343 from the antenna 312.

In some embodiments, one or more of the features shown as separatefunctional blocks can be implemented (“packaged”) on a single chip. Forexample, the body-mountable device 310 can be implemented with therectifier 314, energy storage 316, voltage regulator 318, sensorinterface 321, photodiode 322, and/or the hardware logic 324 packagedtogether in a single chip or controller module. Such a controller canhave interconnects (“leads”) connected to the loop antenna 312 and/orother components (e.g., the photodiode 322 in such case wherein thephotodiode 322 is a separate component form the controller). Such acontroller operates to harvest energy received at the loop antenna 312,measure light received by the photodiode 322 using the sensor interface321, and indicate a signal related to the received light via the antenna312 (e.g., through backscatter radiation 343).

The sensor interface 321 or other electronics related the operation of aphotodetector to receive light from an environment of interest could beconfigured in a variety of ways. In some applications, a very low-powercircuit could be required to optically detect one or more properties ofblood in a portion of subsurface vasculature; for example, to detect avolume or change of volume of blood, an absorption coefficient of theblood in one or more ranges of wavelengths, or some other opticalproperties of the blood and/or portion of vasculature. Such a detectioncould be performed using very low power by relying on the receivinglight emitted from a portion of subsurface vasculature in response toillumination of the portion of subsurface vasculature by, e.g., ambientlight. In such an example detection, a level and/or change in level of aproperty of the received light (e.g., a change in amplitude of thereceived light) could be very small, such that determination of one ormore properties of the portion of subsurface vasculature (e.g., a volumeof blood therein) could include measuring the property of the receivedlight at a high resolution.

Such applications could be enabled by the use of electronics asdescribed herein to operate one or more photodiodes to receive lightfrom a portion of subsurface vasculature (or from some other environmentof interest) and to generate a digital output related to one or moreproperties of the received light. That is, such electronics couldinclude a photodiode voltage source configured to reverse bias aphotodiode such that a current through the photodiode is related to thereceived light, a current mirror configured to output a current relatedto a current through the reverse-biased photodiode, and a sigma-deltamodulator configured to receive the output current of the current mirrorand to generate a digital output related to the output current of thecurrent mirror. Such elements could be configured in a variety of ways.

Further, electronics configured to operate a photodiode as describedherein could include additional elements according to an application. Insome examples, a current-mode digital-to-analog convertor (DAC) could beincluded to sink and/or source a current to the photodiode such that thecurrent received by the current mirror is the photodiode currentadjusted by an amount related to the current sunk and/or sourced by theDAC. For example, such a DAC could be operated to remove a baselinelevel of current through the photodiode that is related to a DC level ofthe light received from a portion of subsurface vasculature, such thatthe current presented to the current mirror is related to a time-varyingaspect of the light received by the photodiode. A sigma-delta modulatoror other elements of the electronics could be used in common betweenmultiple photodiodes (and related electronics, e.g., multiplecorresponding photodiode voltage sources and/or current mirrors), e.g.,by interposing a multiplexer or other electronic switching componentsbetween the sigma-delta modulator and the multiple photodiodes andrelated electronics. Other operations and applications of such a DAC, orof other additional or alternative elements of electronics configured tooperate a photodiode as described herein, are anticipated.

FIG. 4 is a circuit diagram of a photodiode 410 and example electronics400 configured as described herein. The photodiode 410 is configured toreceive light (e.g., visible light, infrared light, ultraviolet light)from an environment of interest (e.g., from a portion of subsurfacevasculature of, e.g., a sclera, eyelid, or other element(s) of an eye)in response to illumination. The illumination could include illuminationby ambient light (e.g., direct or indirect sunlight, light from a lampor lighting fixture, light from a display, light from some other ambientlight directed toward the environment of the environment of interest)and/or by light emitted by a light source in a body-mountable device ofother device that includes the photodiode 410 and example electronics400. The example electronics 400 includes a common electrical ground 405and a voltage supply 407. Elements of the electronics 400 correspondingto electronic elements for operating photodiodes as described elsewhereherein are enclosed by dashed-line boxes. The electronics 400 includecomponents corresponding to a photodiode voltage source 420, a currentmirror 430, and a sigma-delta modulator 440. The electronics 400additionally include a current-mode DAC 450 configured to provide aspecified current 455 to the photodiode 410. The voltage supply 407could be that same as a voltage supply used to power one or more otherelements of the electronics 400 (e.g., operational amplifiers, DACs,comparators, current sinks of 420, 430, 440, 450) than the illustratedelements connected to the voltage supply 407 or could be a separatevoltage supply.

The photodiode 410 could be configured in a variety of ways and includea variety of components/materials. The photodiode 410 could includesilicon, germanium, indium gallium arsenide, lead(II) sulfide, or othermaterials, dopants, or combinations thereof according to an application.The materials and/or configuration of the photodiode 410 and/orassociated components (e.g., filters, mirrors, diffraction gratings,Bragg reflectors/filters, lenses, or other optical elements) could beselected and/or configured such that the photodiode 410 has one or morespecified properties. For example, the photodiode 410 and associatedcomponents could be configured such that a current 415 through thephotodiode 410 when the photodiode voltage source 420 is applying aspecified voltage to reverse-bias the photodiode 410 is related to oneor more properties of light received by the photodiode 410. For example,an amplitude of the current 415 could be related to the intensity of thereceived light within a specified range of wavelengths, where thespecified range of wavelengths is related to the composition andconfiguration (e.g., of filters and of the semiconductor(s) anddopant(s) used to create the photodiode 410, the area of a sensitiveregion of the photodiode 410). In some examples, such a specified rangeof wavelengths could be related to an absorption profile of blood, suchthat an absolute and/or relative volume of blood, an oxygen saturationof blood, or some other property of blood in a portion of subsurfacevasculature could be related to the current 415. In some examples, oneor more properties of a sensitive region of the photodiode 410 could bespecified (e.g., a sensitive area of the photodiode 410 could measureapproximately 700 microns by 700 microns). A dark current, a sensitivityof the current 415 to changes in amplitude or other properties ofreceived light, or other properties of the photodiode 410 could bespecified.

The photodiode voltage source 420 includes an operational amplifier 421whose output is connected to the gate of a first field-effect transistor(FET) 423. The first FET 423 is connected to the voltage source 407, thephotodiode 410, the DAC 450, and the non-inverting input of theoperational amplifier 421 in a negative-feedback topology such theoperational amplifier 421 generates outputs to the first FET 423 suchthat a voltage across the photodiode 410 is approximately equal to afirst reference voltage V_(REF1) applied to the inverting input of theoperational amplifier 421. This includes the output of the operationalamplifier 421 being such that a first FET current 425 flowing throughthe first FET 423 is equal to the photodiode current 415 reduced by anamount substantially equal to the DAC current 455.

Note that the components and connections thereof illustrated in FIG. 4as part of the photodiode voltage source 420 are meant as non-limitingexamples. Other components, circuit topologies, or other configurationsof electronic components could be used to provide a photodiode voltagesource configured to apply a voltage to the photodiode 410 such that thephotodiode 410 is reverse-biased. When reverse biased in this way, thecurrent through the photodiode 410 (e.g., 415) is related to the lightreceived by the photodiode from an environment of interest (e.g., aportion of subsurface vasculature). For example, the first FET 423 couldinstead include one or more other types of transistors (e.g., bipolarjunction transistors (BJTs), metal-oxide-semiconductor field effecttransistors (MOSFETs), junction gate field effect transistors (JFETs))or other electronic elements. Further, the photodiode voltage source 420could include elements other than an operational amplifier (e.g., 421)configured in negative feedback or according to some other topology orscheme to apply a specified voltage to the photodiode 410. For example,the photodiode voltage source 420 could include one or more transistorsconfigured to apply a specified voltage across the photodiode 410.Further, the specified voltage (e.g., V_(REF1)) could be the output of aresistive divider, an ADC, a voltage reference, or some other source ofa specified voltage and could have a voltage value specified accordingto an application (e.g., specified such that the photodiode 410 has aspecified level of current sensitivity to the intensity of receivedlight).

The current mirror 430 is configured to present a current mirror outputcurrent 433 to the sigma-delta modulator 440 that is related to thephotodiode current 415. Current divider 430 includes the first FET 423(that is also part of the photodiode voltage source 420) and a secondFET 431. The gate (i.e., the control terminal) of the second FET 431 isconnected to the gate of the first FET 423 and to the output of theoperational amplifier 421. The second FET 431 is additionally connectedto the voltage source 407 and the sigma-delta modulator 440 such thatthe current mirror output current 433 is related to the first FETcurrent 425 and thus to the photodiode current 415. Note that othertypes of transistor (e.g., BJTs, MOSFETs, JFETs) could be used insteadof or in combination with the illustrated first and second FETs 423,431. Further the configuration of a current mirror used to present acurrent to a sigma-delta modulator that is related to the currentthrough a photodiode (e.g., 440 and 410, respectively) could includeadditional or alternate components configured in a similar or differenttopology from that shown here.

Note that a relationship between the current mirror output current 433and the first FET current 425 could be related to the configuration ofthe current mirror 430. Specifically, a ratio between the current mirroroutput current 433 and the first FET current 425 (i.e., a transfer ratioof the current mirror 430) could be related to a ratio betweenproperties and/or measurements of features of the corresponding firstand second FETs 423, 431. For example, the transfer ratio of the currentmirror 430 could be related to a ratio of the widths of thecorresponding first and second FETs 423, 431. In some examples, such aratio could be specified to one (i.e., a transfer ratio of unity) byspecifying that the characteristics of the first and second FETs 423,431 (e.g., channel width, temperature, gate properties, semiconductorproperties) be substantially identical. This could be accomplished byforming the first and second FETs 423, 431 near to each other on thesame semiconductor chip or wafer. Alternatively, the first and secondFETs 423, 431 could be formed on the same wafer and having substantiallythe same configuration excepting one property (e.g., a channel width)that is different between the first and second FETs 423, 431 such thatthe transfer ratio of the current mirror 430 is some specified valueother than unity. Other properties of other types of transistors used toform a current mirror (e.g., gate width of BJTs) could be similarlyspecified to achieve a current mirror having a specified transfer ratio.

The sigma-delta modulator 440 is configured to receive an input (e.g.,current mirror output current) and to provide a digital output (OUTPUT)related to the input. As illustrated in FIG. 4, example sigma-deltamodulator 440 includes a capacitor 441, a comparator 443, and a switchedcurrent sink comprising an electronic switch 445 and a current sink 447.The capacitor 441 is connected between the common electrical ground 405and the input to the sigma-delta modulator 440 such that the capacitor441 is charged by the input current (i.e., by 433). The comparator 443is configured to compare the voltage across the capacitor 441 to areference voltage V_(REF2) and to generate a digital pulse when thevoltage across the capacitor 441 is greater than V_(REF2). The output ofthe comparator 443 is used as the OUTPUT of the sigma-delta modulator.The output of the comparator 443 is further used to operate theelectronic switch 445 of the switched current sink such that each pulseoutput from the comparator 443 causes the current sink 447 to dischargea specified current from the capacitor 441 such that the capacitor 441is discharged by a specified amount (e.g., by a specified amount ofcharge) for each digital pulse output by the comparator 443.

The comparator 443 could be clocked such that digital output pulses ofthe comparator are only generated at specified points in time relativeto a clock signal provided to the comparator 443 (e.g., by a controller,a multivibrator, a quartz oscillator, or some other clock source).Parameters of operation of the sigma-delta modulator 440 (e.g., thevoltage value of V_(REF2), a clock frequency of a clock signal appliedto the comparator 443, a width and/or duty cycle of digital pulsesoutput by the comparator 443, a level of current sunk by the currentsink 447) could be specified and/or changed such that the digital OUTPUTof the sigma-delta modulator 440 was related to the photodiode current415 with a specified resolution, sample frequency, oversample frequency,or other property related to an application. For example, V_(REF2) couldbe the output of an ADC or some other controllable voltage source andcould be adjusted (e.g., lowered) to adjust a resolution (e.g., toincrease a resolution) of the sigma-delta modulator. In another example,the current sunk by the current source 447 could be a specified value(e.g., 5 nanoamps) related to a resolution of the sigma-delta modulator440, and could further be controllable (e.g., could be controlledbetween two or more specified values of sunk current) to control theresolution of the sigma-delta modulator 440.

In an example wherein the photodiode 410 is configured to receive lightfrom a portion of subsurface vasculature and the digital output of theelectronics 400 is used to determine a pulse rate of blood in theportion of subsurface vasculature. The configuration of the electronics400 (e.g., a value of V_(REF2), a current sunk by the current sink 447,a pulse width and/or clock frequency of the comparator 443) could bespecified to allow determination of the frequency of the pulse ratewithin a specified accuracy using the digital output of the sigma-deltamodulator 440. For example, an effective sample rate of the sigma-deltamodulator 440 equal to or greater than 1 kilohertz in order to determinea pulse rate with greater than one beat-per-minute accuracy up to apulse rate of 240 beats-per-minute.

In order to achieve a specified effective sample rate, a clock frequencyapplied to the comparator could be specified to be greater than theeffective sample rate multiplied by a factor relating to a desirednumber of bits of resolution of the output relative to the input of themodulator 440. For example, an effective sample rate of 50 Hertz and aresolution of 12 bits could specify that the clock frequency applied tothe sigma-delta modulator 440 be greater than 50*2^12=204800 Hertz.Further, parameters of operation of the electronics 400 (e.g., V_(REF1),V_(REF2), a transfer ratio of the current mirror 430, the value ofcurrent sunk by the current sink 447, a pulse width of pulses output bythe comparator 443) could be specified relative to a current resolutionof the photodiode current 412 specified according to an application. Forexample, the electronics 400 could be configured to generate a digitaloutput that is related to the current through the photodiode 410 (i.e.,415) with a resolution that is less than approximately 100 picoamps.

Note that the configuration and connection of components of thesigma-delta modulator 440 as illustrated in FIG. 4 is meant as anon-limiting example. Additional or alternative components configured ina similar or different topology from that shown here are anticipated toprovide a sigma-delta modulator configured to generate a digital output(e.g., OUTPUT) related to an input current (e.g., 433). Further asigma-delta modulator (e.g., 440) could be configured to generate adigital output related to the current through multiple photodiodes(e.g., 410). For example, a plurality of photodiodes and correspondingplurality of respective capacitors, photodiode voltage sources, andcurrent mirrors (configured similarly or differently to, e.g., 441, 420,430, respectively) could be configured to input current to a singlesigma-delta modulator (e.g., 440). In a particular example, amultiplexer or other electronic component could be interposed betweencomponents of the sigma-delta modulator and the plurality ofphotodiodes, photodiode voltage sources, current mirrors, capacitors,and/or other electronic components. The digital output of such amultiplexed sigma-delta modulator could thus correspond to a pluralityof photodiode currents of the respective plurality of photodiodesaccording to a time division multiplexing scheme or according to someother relationship. In some embodiments, two or more photodiodes couldbe configured to received light from the same portion of subsurfacevasculature having wavelengths within respective two or more ranges ofwavelengths (e.g., ranges of wavelengths approximately equal to 700nanometers and 900 nanometers, respectively) and the digital output,related to the lights received from the two or more photodiodes, couldbe used to determine an oxygen saturation or other information aboutblood in the portion of subsurface vasculature.

The DAC 450 could be configured in a variety of ways familiar to oneskilled in the art. For example, the DAC 450 could include an R-2Rladder, a binary-weighted DAC, an amplifier, a pulse width modulator, adelta-sigma modulator, a successive-approximation DAC, or some othercomponents or combination thereof configured to source and/or sink aspecified controllable DAC current 455. In some examples, the DAC 450could include an electronic switch connected in series with a resistorhaving a specified resistance. The series combination of the electronicswitch and the resistor could be connected between the photodiode (asshown in FIG. 4) and the common electrical ground 405 or to some otherspecified voltage relative to the common electrical ground 405. Theelectronic switch could be operated according to a pulse widthmodulation scheme or according to some other scheme to source and/orsink a specified DAC current 455 to/from the photodiode 410.Instantaneous and/or mean values of current specified to control the DAC450 to produce DAC current 455 could be determined in a variety of waysbased on a variety of factors. In some examples, the DAC 450 could beoperated to remove a low-frequency offset from the input (e.g., 433)received by the sigma-delta modulator by injecting a current (e.g., 455)related to the low-frequency offset into the photodiode 410.

The digital output (OUTPUT) of the electronics 400 could be used in avariety of ways according to an application. In some examples, thedigital output could be decimated, e.g., specified time periods (e.g., aplurality of time periods having durations equal to a specified numberof clock cycles) of the digital output could be used to generaterespective digital output values by, e.g., summing the number of pulsesin each of the time periods. In this way, a plurality of digital valuesrelated to the current through the photodiode 410 during a respectiveplurality of time periods could be determined. The digital output(OUTPUT) could be transmitted to some other system (e.g., a controllerof a device including the electronics; to another system incommunication with such a device); additionally or alternatively, asignal or value (e.g., a digital value determined through decimation ofthe OUTPUT) could be transmitted to some other system. A signal or value(e.g., a decimated digital value) based on the OUTPUT could be used todetermine one or more properties of an environment that emitted thelight received by the photodiode 410 (e.g., a volume or some otherinformation about blood in a portion of subsurface vasculature).Additionally or alternatively, some other information (e.g., timinginformation) about the OUTPUT could be used to determine one or moreproperties of an environment that emitted the light received by thephotodiode 410. For example, an autocorrelation, a spectrogram, aFourier transform, or some other operation could be performed based on asequence of digital pulses in OUTPUT and used to determine someinformation (e.g., a rate and/or phase of heart beats of a heart pumpingblood through a portion of subsurface vasculature).

The electronics 400 could be configured to generate a digital outputwhile consuming very little power. For example, photodiodes and relatedelectronics disclosed herein could be included in a low-powerenvironmental sensor that is part of a distributed sensor network, alow-power body-mountable (e.g., eye-mountable) device, or some otherdevice having access to a limited power budget (e.g., having alow-capacity battery, being configured to scavenge power fromenvironmental sources, being configured to be powered by receivedelectromagnetic radiation from a reader or from some other source). Insuch examples, the electronics 400 and photodiode 410 could beconfigured to operate while using less than approximately 1 microamp, orpreferentially less than approximately 500 nanoamps.

IV. Example Methods

FIG. 5 is a flowchart of a method 500 for operating a light sensor in abody-mountable device to measure light (e.g., visible light, infraredlight, ultraviolet light) received from a portion of subsurfacevasculature of a body (e.g., of a cornea, iris, sclera, eyelid, or otheraspect of an eye of the body). The body-mountable device includes (i) aphotodiode configured to detect the light received from the portion ofsubsurface vasculature, (ii) a photodiode voltage source configured toapply a voltage to the photodiode such that the photodiode is reversebiased and such that a current through the photodiode is related to thereceived light, (iii) a sigma-delta modulator configured to receive aninput and to provide a digital output related to the input, and (iv) acurrent mirror configured to provide an output current related to thecurrent through the photodiode and to present the output current as aninput to the sigma-delta modulator. The light received by the photodiodeis related to blood in the portion of subsurface vasculature (e.g., to avolume, extinction coefficient, flow rate, pulse rate, oxygensaturation, or some other property or properties of the blood). Thebody-mountable device and elements thereof could be configured and/oroperated as described herein. Further, the body-mountable device couldinclude additional components configured to provide some functionality.For example, the body-mountable device could include a controllerconfigured to operate elements of the body-mountable device and/or todetermine a property of the blood based on the digital output provide bythe sigma-delta modulator, an antenna configured to receivedelectromagnetic energy to power the body-mountable device and/or toindicate information (e.g., to indicate the digital output and/or somesignal determined from such) wirelessly, an electrochemical sensor, anaccelerometer, or some other components or combinations thereof.

The method 500 includes mounting the body-mountable device to anexternal body surface proximate to a portion of subsurface vasculature(502). In some examples, the body-mountable device could be formed tosubstantially conform to a cornea of an eye of the body, and the devicecould be mounted on the cornea such that the photodiode receives lightfrom a portion of subsurface vasculature of the cornea, iris, sclera,eyelid, or other aspect of the eye. The body-mountable device could bemounted to the eye such that the photodiode receives light from aspecified portion of subsurface vasculature and/or a portion ofsubsurface vasculature in a specified portion, aspect, or region of theeye (e.g., a region of the eye lateral to the pupil of the eye); that isthe body-mountable device could be emplaced on the eye such that alocation and/or orientation of the body-mountable device relative to theeye is specified. The body-mountable device could be configured suchthat motions, features, or other aspects or processes of the eye causethe body-mountable device to be aligned with (i.e., to have a specifiedlocation and/or orientation relative to) the eye; for example, thebody-mountable device could be weighted, or could have a shape thatinteracted with elements of the eye (e.g., a flattened edge that isaligned by eyelid motion of the eye) such that the device is aligned.

The method 500 includes operating the body-mountable device to generatethe digital output (504). This could include applying a voltage (i.e.,applying power) to the electronics (e.g., photodiode voltage source,current mirror, and sigma-delta modulator, among other components). Thiscould include setting one or more parameters or controls of theelectronics (e.g., set or reference voltages, clock frequencies, currentsink values) such that the digital output has one or more specifiedproperties (e.g., an effective sample rate, a resolution) relative tothe current through the photodiode and/or to one or more properties ofthe light received from the portion of subsurface vasculature. Operatingthe body-mountable device to generate the digital output (504) couldinclude any process or operation as described herein for operatingelectronics (e.g., 400, other electronics herein) to generate a digitaloutput that is related to light received by the photodiode. Operatingthe body-mountable device to generate the digital output (504) couldinclude operating elements (e.g., light-emitting diode(s) (LED(s))) ofthe body-mountable device to generate a light (e.g., a visible light, aninfrared light, an ultraviolet light, light having a specified spectralprofile) to illuminate the portion of subsurface vasculature.Additionally or alternatively, light received by the photodiode from theportion of subsurface vasculature could be received in response toillumination of the portion of subsurface vasculature by ambient lightsources.

The method 500 further includes determining a property of blood in theportion of subsurface vasculature based on the generated digital output(506). In some examples, this could include a controller or otherprocessor of the body-mountable device performing the determination;additionally or alternatively, the body-mountable device could indicatea signal (e.g., using an antenna of the body-mountable device) relatedto the digital output (e.g., the digital output, a decimated versionthereof, some other signal based on the digital output), and some othersystem (e.g., a reader configured to wirelessly communicate with and/orpower the body-mountable device) could receive the indicated signal anddetermine a property of blood in the portion of subsurface vasculaturebased on the received signal. Determined properties of the blood couldinclude a blood volume, a blood flow rate, a pulse rate, an extinctionor absorption coefficient, an oxygen saturation, or some other propertyof the blood.

The method 500 could include additional steps or elements in addition tothose depicted in FIG. 5 (i.e., 502, 504, 506). For example, the method500 could include detecting light received from the portion ofsubsurface vasculature at a plurality of points in time and determined aproperty of blood (e.g., a property of blood flow) in the portion ofsubsurface vasculature based on the plurality of detected receivedlights. For example, an intensity of the received light could bedetected at a plurality of points in time and used to determine arespective plurality of volumes of blood in the portion of subsurfacevasculature. A blood flow, a pulse rate, a blood flow profile, or someother information about the blood in the portion of subsurfacevasculature could be determined based on the plurality of detectedreceived lights and/or the plurality of determined blood volumes. Inanother example, the photodiode could be configured to receive lightwithin a first range of wavelengths and the body-mountable device couldinclude further photodetector configured to detect light form theportion of subsurface vasculature in a second range of wavelengths. Themethod 500 could include operating the further photodetector to detectthe light in the second range of wavelengths and determining an oxygensaturation of blood in the portion of subsurface vasculature based onthe detected received lights in the first and second ranges ofwavelengths (e.g., based on a ratio of the intensities of the lights andabsorption spectra of oxygenated and deoxygenated blood within the firstand second ranges, or according to some other relationship operation).Other additional elements of the method 500 are anticipated.

CONCLUSION

Where example embodiments involve information related to a person or adevice of a person, the embodiments should be understood to includeprivacy controls. Such privacy controls include, at least, anonymizationof device identifiers, transparency and user controls, includingfunctionality that would enable users to modify or delete informationrelating to the user's use of a product.

Further, in situations in where embodiments discussed herein collectpersonal information about users, or may make use of personalinformation, the users may be provided with an opportunity to controlwhether programs or features collect user information (e.g., informationabout a user's medical history, social network, social actions oractivities, profession, a user's preferences, or a user's currentlocation), or to control whether and/or how to receive content from thecontent server that may be more relevant to the user. In addition,certain data may be treated in one or more ways before it is stored orused, so that personally identifiable information is removed. Forexample, a user's identity may be treated so that no personallyidentifiable information can be determined for the user, or a user'sgeographic location may be generalized where location information isobtained (such as to a city, ZIP code, or state level), so that aparticular location of a user cannot be determined. Thus, the user mayhave control over how information is collected about the user and usedby a content server.

The particular arrangements shown in the Figures should not be viewed aslimiting. It should be understood that other embodiments may includemore or less of each element shown in a given Figure. Further, some ofthe illustrated elements may be combined or omitted. Yet further, anexemplary embodiment may include elements that are not illustrated inthe Figures.

Additionally, while various aspects and embodiments have been disclosedherein, other aspects and embodiments will be apparent to those skilledin the art. The various aspects and embodiments disclosed herein are forpurposes of illustration and are not intended to be limiting, with thetrue scope and spirit being indicated by the following claims. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in thefigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which arecontemplated herein.

What is claimed is:
 1. A body-mountable device comprising: a photodiode,wherein the photodiode is positioned to detect light received from aportion of subsurface vasculature, wherein the light received from theportion of subsurface vasculature is related to blood in the portion ofsubsurface vasculature; a photodiode voltage source, wherein thephotodiode voltage source applies a voltage to the photodiode such thatthe photodiode is reverse biased, wherein a photodiode current throughthe photodiode is related to the light received from the portion of subsurface vasculature; a current-mode digital-to-analog converter (DAC),wherein the current-mode DAC sources or sinks a DAC current; a currentmirror, wherein the current mirror has an input current path and anoutput current path, wherein the current mirror provides an outputcurrent in the output current path that is related to an input currentin the input current path, wherein the photodiode and current-mode DACare connected in parallel to the input current path of the currentmirror such that the input current is related to the photodiode currentand DAC current; and a sigma-delta modulator coupled to the outputcurrent path of the current mirror, wherein the sigma-delta modulatorprovides a digital output related to the output current in the outputcurrent path of the current mirror.
 2. The body-mountable device ofclaim 1, wherein the body-mountable device further comprises: a shapedpolymeric material, wherein the shaped polymeric material has a concavesurface and a convex surface, wherein the concave surface can beremovably mounted on an eye and the convex surface is opposite theconcave surface; a substrate at least partially embedded within theshaped polymeric material, wherein the photodiode, photodiode voltagesource, and sigma-delta modulator are disposed on the substrate; anantenna disposed on the substrate, wherein the antenna is operable toindicate a signal related to the digital output of the sigma-deltamodulator.
 3. The body-mountable device of claim 2, wherein thephotodiode is directed toward the concave surface of the shapedpolymeric material such that the portion of subsurface vasculature is aportion of vasculature of the eye when the concave surface is mounted onthe eye.
 4. The body-mountable device of claim 3, wherein the photodiodeis disposed on the substrate such that the portion of subsurfacevasculature is a portion of subsurface vasculature that is disposed in alateral region of the eye relative to a pupil of the eye when theconcave surface is mounted on the eye.
 5. The body-mountable device ofclaim 2, wherein the photodiode is directed toward the convex surface ofthe shaped polymeric material such that the portion of subsurfacevasculature is a portion of vasculature of an eyelid of the eye when theconcave surface is mounted on the eye and the eyelid is at leastpartially closed over the eye.
 6. The body-mountable device of claim 1,wherein the photodiode current through the photodiode is related to anintensity of the light received from the portion of subsurfacevasculature, and wherein the intensity of the light is related to avolume of the blood in the subsurface vasculature.
 7. The body-mountabledevice of claim 1, wherein the current mirror comprises a firsttransistor and a second transistor, wherein the first transistor is apart of the photodiode voltage source and is electronically connectedbetween a voltage source and the photodiode, wherein the photodiodevoltage source is configured to operate the first transistor by applyinga control signal to a control terminal of the first transistor such thata voltage across the photodiode is substantially equal to a specifiedvoltage, wherein the second transistor is electronically connectedbetween the voltage source and the sigma-delta modulator, and whereinthe control signal is additionally applied to a control terminal of thesecond transistor.
 8. The body-mountable device of claim 1, wherein thesigma-delta modulator comprises: a capacitor, wherein the capacitor iscoupled to the output current path of the current mirror; a switchedcurrent sink, wherein the switched current sink can be operated todischarge a specified current from the capacitor; and a comparator,wherein the comparator compares a voltage across the capacitor to aspecified reference voltage, wherein the comparator generates a digitalpulse when the voltage across the capacitor is greater than thespecified reference voltage, and wherein the switched current sink isoperated by the digital pulses generated by the comparator such that theswitched current sink discharges the capacitor by a specified amount foreach generated digital pulse.
 9. The body-mountable device of claim 1,wherein the digital output of the sigma-delta modulator is related tothe photodiode current through the photodiode with a resolution that isless than approximately 100 picoamps.
 10. The body-mountable device ofclaim 1, further comprising an antenna, wherein the antenna is operableto receive radio frequency energy to power the body-mountable device.11. The body-mountable device of claim 1, further comprising an antenna,wherein the antenna is operable to wirelessly indicate a signal relatedto the digital output of the sigma-delta modulator.
 12. Thebody-mountable device of claim 1, wherein the photodiode current throughthe photodiode is related to an intensity of the light received from theportion of subsurface vasculature within a first range of wavelengths,wherein the body-mountable device further includes a photodetector thatcan detect an intensity of light received from the portion of subsurfacevasculature within a second range of wavelengths, and wherein theintensity of the light detected by the photodiode and the intensity ofthe light detected by the photodetector are related to an oxygensaturation of blood in the portion of sub surface vasculature.
 13. Amethod, comprising: receiving, by a photodiode in a body-mountabledevice, light from a portion of subsurface vasculature, wherein thelight received from the portion of subsurface vasculature is related toblood in the portion of subsurface vasculature; applying, by aphotodiode voltage source in the body-mountable device, a voltage to thephotodiode such that the photodiode is reverse biased, wherein aphotodiode current through the photodiode is related to the lightreceived from the portion of subsurface vasculature; providing, by acurrent-mode digital-to-analog converter (DAC) in the body-mountabledevice, a DAC current; providing, by a current mirror in thebody-mountable device, an output current in an output current path ofthe current mirror, wherein the output current is related to an inputcurrent in an input current path of the current mirror, and wherein thephotodiode and current-mode DAC are connected in parallel to the inputcurrent path of the current mirror such that the input current isrelated to the photodiode current and DAC current; and generating, by asigma-delta modulator in the body-mountable device, a digital outputrelated to the output current in the output current path of the currentmirror.
 14. The method of claim 13, further comprising: determining aproperty of blood in the portion of subsurface vasculature based on thegenerated digital output.
 15. The method of claim 14, wherein thephotodiode current through the photodiode is related to an intensity ofthe light received from the portion of subsurface vasculature within afirst range of wavelengths, wherein the body-mountable device furtherincludes a photodetector that can detect an intensity of light receivedfrom the portion of subsurface vasculature within a second range ofwavelengths, wherein the method further includes operating thephotodetector to detect the intensity of light received from the portionof subsurface vasculature within the second range of wavelengths, andwherein determining the property of blood in the portion of subsurfacevasculature based on the generated digital output comprises determiningan oxygen saturation of blood in the portion of subsurface vasculaturebased on the generated digital output and the intensity of lightdetected using the photodetector.
 16. The method of claim 14, whereinthe photodiode current through the photodiode is related to an intensityof light received from the portion of subsurface vasculature, whereinthe intensity of the light received from the portion of subsurfacevasculature is related to a volume of the blood in the subsurfacevasculature, wherein determining the property of blood in the portion ofsubsurface vasculature comprises determining the volume of blood in theportion of subsurface vasculature based on the digital output generatedby the sigma-delta modulator, wherein determining the property of bloodin the portion of subsurface vasculature based on the generated digitaloutput is performed a plurality of times at a plurality of respectivepoints in time to generate a plurality of respective determined volumesof blood in the portion of subsurface vasculature, and furthercomprising: determining a property of blood flow in the portion ofsubsurface vasculature based on the plurality of determined volumes ofblood in the portion of subsurface vasculature.
 17. The method of claim13, wherein the body-mountable device further comprises: (i) a shapedpolymeric material, wherein the shaped polymeric material has a concavesurface and a convex surface, wherein the concave surface can beremovably mounted on an eye and the convex surface is opposite theconcave surface, and (ii) a substrate at least partially embedded withinthe shaped polymeric material, wherein the photodiode, photodiodevoltage source, current-mode DAC, current mirror, and sigma-deltamodulator are disposed on the substrate.
 18. The method of claim 13,wherein the body-mountable device further comprises an antenna, andfurther comprising: indicating, using the antenna, a wireless signalrelated to the digital output of the sigma-delta modulator.
 19. Themethod of claim 18, wherein the indicating comprises reflecting aradio-frequency signal received by the antenna.
 20. The method of claim13, wherein the body-mountable device further comprises an antenna,further comprising: using a radio-frequency signal received by theantenna to power the body-mountable device.